1. Field of the Invention
The present invention relates to a thermoelectric material, and in particular, to a thermoelectric material comprising, as a major phase, a half Heusler compound having an MgAgAs type crystal structure. The present invention also relates to a thermoelectric element formed by using of this thermoelectric material.
2. Description of the Related Art
In recent years, concomitant with the awareness of issues with respect to global environmental problems, there is increasing concern about a thermoelectric cooling element utilizing Peltier effect for achieving flon-less cooling. Likewise, there is also increasing concern about a thermoelectric generating element which is capable of directly converting unutilized waste heat energy into electric energy for the purpose of minimizing the quantity of carbon dioxide discharged into the atmosphere, in view of overcoming the problem of global warming.
As for the p-type or n-type thermoelectric cooling materials and thermoelectric power-generating materials to be employed for the manufacture of the thermo-electric elements, materials having a Bi—Te-based monocrystalline or polycrystalline structure are widely employed because of their excellent conversion efficiency. Even in the case of the thermoelectric materials to be employed under high-temperature conditions higher than room temperature, Pb—Te-based materials are employed for any of these p-type or n-type thermoelectric cooling materials and thermoelectric power-generating materials.
Pb (lead) included in the Pb—Te-based materials is noxious and hazardous to the human body and also undesirable in view of the global environmental problem. In the Bi—Te-based materials, Se is generally included as an impurity, which is also toxic to the human body. In view of the global environmental problem also, the inclusion of Se is undesirable. Te, which is employed in these material systems, is very scarce in deposits in the earth and hence it is difficult to supply it in sufficient amounts. Therefore, it is greatly desired to develop a thermoelectric material which is higher in conversion efficiency as compared with the aforementioned Bi—Te-based materials or Pb—Te-based materials, and is harmless to the human body.
The half Heusler compounds can be represented by a chemical formula ABX and is an intermetallic compound having an MgAgAs type cubic crystal structure wherein the B atom is inserted into the NaCl type crystal lattice of AX. The compounds having a structure of this type exhibit a high Seebeck coefficient at room temperature. For example, it is reported that TiNiSn exhibits a Seebeck coefficient of −142 μV/K, ZrNiSn exhibits −176 μV/K, and HfNiSn exhibits −124 μV/K.
Incidentally, the performance index Z of the thermoelectric material can be represented by the following formula.Z=α2σ/κ  (1)
In this formula (1), α is the Seebeck coefficient of thermoelectric material; σ is electric conductivity; and κ is thermal conductivity. The inverse number of electric conductivity can be represented by electrical resistivity ρ.
Z may have a dimension which is an inverse to temperature, and when this performance index Z is multiplied by an absolute temperature, it becomes a dimensionless number. Namely, this dimensionless number ZT is called “a dimensionless figure-of-merit” and is correlated with the thermoelectric conversion efficiency of thermoelectric materials in such a way that the larger the value of this ZT of the materials becomes, the higher the thermoelectric conversion efficiency will be realized by the materials. Namely, as the materials become more difficult in transmitting heat, but become easier in transmitting electricity, enabling the materials to exhibit a larger thermo-electromotive force, the materials become a thermoelectric material which is capable of exhibiting a higher thermoelectric conversion efficiency. For example, in the case of the Bi—Te-based materials which are known to exhibit the highest dimensionless figure-of-merit among the known thermoelectric materials, the dimensionless figure-of-merit thereof is about 1.0 at a temperature of 300K.
Although the aforementioned half Heusler compound ZrNiSn is capable of exhibiting a Seebeck coefficient of as high as −176 μV/K at room temperature, the electrical resistivity thereof at room temperature is as high as 11 mΩcm and still more, the heat conductivity thereof is as high as 8.8 W/mK. As a result, it is reported that the dimensionless figure-of-merit ZT of the ZrNiSn is as small as 0.010 and hence the thermoelectric conversion efficiency thereof is also small. In the cases of TiNiSn and HfNiSn, the thermoelectric conversion efficiency thereof is more inferior, i.e. about 0.007 for TiNiSn and 0.005 for HiNiSn.
Meanwhile, as for the half Heusler compound containing a rare earth element, there is known for instance HoPdSb. The Seebeck coefficient of HoPdSb is 150 μV/K at room temperature. Although the heat conductivity of HoPdSb is 6 W/mK, which is slightly smaller than that of the ZrNiSn, the electrical resistivity thereof at room temperature is as high as 9 mΩcm and hence the dimensionless figure-of-merit ZT of HoPdSb is only 0.01. It is also reported that the dimensionless figure-of-merit at room temperature of Ho0.5Er0.5PbSb1.05, Er0.25Dy0.75Pb1.02Sb and Er0.25Dy0.75PbSb1.05 is 0.04, 0.03 and 0.02, respectively.
The present invention has been achieved in view of the aforementioned problems and hence, one object of the present invention is to provide a thermoelectric material comprising as a major phase, a half Heusler compound, this thermoelectric material being capable of exhibiting a high dimensionless figure-of-merit ZT while making it possible to sufficiently suppress the heat conductivity and to maintain a high Seebeck coefficient and a low electric resistivity. Another object of the present invention is to provide a thermoelectric element obtained by using such a thermoelectric material.